1. Field of the Invention
The present invention relates to an image reading apparatus for reading an original image by photoelectric conversion and thereby obtaining an electrical image signal.
2. Related Background Art
An image reading apparatus, or image scanner, for reading image information (for example of documents or books in the form of an electrical information signal), is generally provided with, as shown in FIG. 1, a light source 4 for illuminating an original document 3 placed on a support glass plate 2 of a casing 1; three movable mirrors 8, 9, 10 for guiding the reflected light 5 from said original 3 to a line sensor 7 through an imaging lens 6; a circuit board 12 for processing the electrical signal obtained by photoelectric conversion in said line sensor 7 and for controlling the function of the entre apparatus; and an interface (I/F) unit 13 for sending the image signal to external equipment. The light source 4 and the mirror 8 move integrally with a speed equal to twice of that of the mirrors 9, 10.
The light source 4 for illuminating the original 3 is often composed of a fluorescent lamp because of the sufficient light intensity and the low cost. However the fluorescent lamp is also associated with certain drawbacks, such as the necessity for a preheating time for preventing the deterioration of electrodes, and the necessity for high frequency lighting in order to avoid unevenness in the illumination. The most serious drawback, however, of the fluorescent lamp is the significant dependence of light intensity on the ambient temperature as shown in FIG. 2, and the variation in light intensity according to the lighting time. Various measures have been employed for avoiding these drawbacks.
One of such conventional measures is to attach a planar heater to the fluorescent lamp, for heating the tube wall of said lamp at the start of lighting and when in a low temperature state. However, such a method is still unsatisfactory in the overall performance, requiring considerable electric power for the heater and involving a time lag before a sufficient effect is obtained.
Another measure lies in the control of the current in the fluorescent lamp, according to the amount of light detected from the lamp. However, this method requires a photosensor for said detection, and involves a considerable cost increase for example in the power source, despite the fact that the light intensity does not increase in proportion to the electric power supplied to the fluorescent lamp.
Still another measure, with relatively limited cost increase, employs a circuit as shown in FIG. 3. In such conventional method, around an original reading area 2A on the glass plate 2, there are provided, as shown in FIG. 4, a first reference white plate 15 positioned along the main scanning direction MD of the line sensor 7 to be read by said line sensor 7 immediately before the reading of the original image, and second reference white plate 16 positioned along the sub scanning direction SD to be read by the line sensor 7 during the reading of the original image. Prior to the original image reading, the line sensor 7 provides signals as shown in FIG. 5 corresponding to the reference white plates 15, 16, wherein an area A1 indicates the signal obtained by reading the plate 15, while an area A2 indicates the signal obtained by reading the plate 16.
It is assumed, in FIG. 5, that the reference signal level of the area A2 is a half of the maximum signal level of the area A1.
Referring to FIG. 3, a signal of the area A2 is sampled by an analog switch 17 and is doubled by an amplifier 18 to obtain a voltage V1, which is subjected to sample holding by a sample-and-hold (S/H) circuit 19. Thus processed signal is supplied to a reference terminal of an A/D converter 20 through AS2 of analog switches 23. Also, an input video signal obtained by reading the reference white plate 15 is supplied, through an amplifier 40, to the A/D converter 20, then digitized utilizing the signal from the S/H circuit 19, and stored temporarily as a reference signal in a memory 21.
Thereafter said reference signal is released from said memory 21, and is supplied to a D/A converter 22 to obtain an analog signal which is supplied to the reference terminal of the A/D converter 20 through AS3 of the analog switches 23. Then, the input video signal obtained by reading the original image is digitized in the A/D converter 20, using the signal read from the memory 21 as reference. At the same time the reference signal varying in the sub scanning direction is supplied to the reference terminal of the D/A converter 22 through an analog switch 17, an amplifier 18 and a S/H circuit 19, thereby compensating the fluctuation in time.
However, in such circuit structure as shown in FIG. 3, if a fluorescent lamp is used as the light source, the level of the input video signal varies about ten times or more at maximum. Consequently, when the light intensity of the light source is low, the reference voltage of the A/D converter 20 also becomes low, so that the precise digitization of the input video signal cannot be expected in comparison with the case in the presence of a reference voltage of a sufficient level. Also, since the shading correction is based on the same reference voltage, the precision of the shading correction is lowered when the light intensity of the light source is weak, and this fact also leads to a lowered precision of digitization.